Patent GB-1215711-A (GB)
Title: Improvements relating to control systems for grab hoists
Inventor: JONES ERIC FRANCIS
IPC: B66C13/26
Filed: 1967-01-04
Abstract: 1,215,711. Control of two A.C. motors. MTE-CONTACTOR (SYSTEMS) Ltd. 4 Jan., 1968 [4 Jan., 1967], No. 421/67. Heading H2J. In a grab hoist, the holding-line and closing- line winches are driven by respective induction motors HM, CM, each having an individual controller HMC, CMC. The controller HMC has at least one setting to provide countercurrent lowering which limits the speed of the grab under its own weight, whilst the controller CMC has a first power-lowering setting adapted to provide a drive-down torque subject to automatic speed control operative to opencircuit the motor at a predetermined speed less than synchronous speed. Each motor has a rotor voltage-sensitive relay HVR, CVR, controlling contactors HRC1-HRC4 or CRC1-CRC4 which operate in sequence to adjust the rotor resistance and are associated with calibration resistance networks. The controllers are interconnected so that the mode of operation for certain settings of a master switch is conditional upon the setting of the other. A jamming relay JR responds to overload and reduces the torque of the motor CM by opening rotor contactors until the load is shared by the motor HM. When the controller HMC is moved to its first hoisting notch, the controller CMC being on a similar notch, contactor HHC is closed and relay HVR responds to close contactors HRC1-HRC4 in sequence. As the motor accelerates, the rotor voltage decreases to open the rotor contactors and maintain the speed within a narrow range. On the second notch, the contactors HRC1-HRC4 are closed permanently and, thereafter, contactors HRC5- HRC7 reduce the rotor resistance in a timed sequence. If the controller HMC is moved to the " off " position from any hoisting notch, the contactor HHC opens, contactor HLC closes, and the calibration of the relay HVR is modified so that it remains energized until the motor speed approaches zero. Rotor contactors HRC1-HRC4 are held closed and the motor is decelerated by counter-current braking. At zero speed, the contactor HLC opens and an electromagnetic brake is applied. When the controller HMC is moved from the " off" position to the first lowering notch, the contactor HHC is closed for lowering under counter-current braking operation, the resistors r4, r5 being short-circuited to re-calibrate the relay HVR. Initially, the rotor circuit of the motor HM is open-circuited and the grab accelerates under its own weight until relay HVR becomes energized to operate contactors HRC1-HRC4 in sequence to and maintain the speed within a narrow range. On moving the controller HMC to the second lowering notch, the relay HVR is further modified to allow for operation at a higher speed. On the third notch, power lowering is provided, contactors HHC, HLC being respectively opened and closed, whilst contactors HRC1-HRC5 are all closed. The relay HVR is ineffective, and the motor HM operates with dynamic braking above synchronous speed. When the controller HMC is moved to the " off " position from any lowering notch, the motor HM is subjected to counter-current braking until the relay HVR is de-energized at zero speed. The controller CMC is generally similar to the controller HMC except that it has an extra fourth lowering notch and a conditional mode of operation when set to the second lowering notch. The calibration network for the relay CVR comprises resistors r1-r7. If the controllers HMC, CMC are respectively moved to their first and third hoisting notches, the effective setting of the controller CMC remains as it is on the second hoisting notch to prevent the closing-line running up to full speed and taking substantially the whole load. On the second lowering notch of the controller CMC, the mode of operation depends on the relative position of the controller HMC. If the latter is on its second or third lowering notch, contactor CRC1 is brought within the control of the relay CVR in addition to the contactors CRC2- CRC4, and normal operation is obtained with counter-current braking under voltage relay control. If, however, the controller HMC is on its first lowering notch, relay R is energized to establish lowering connections for the motor CM, the relay CVR being re-calibrated to be energized at a low speed. The motor CM provides a drive-down torque until the speed increases to the sub-synchronous value at which the relay CVR is de-energized and the rotor circuit opens to remove the driving torque. On the second lowering notch of the controller HMC, if the controller CMC is still on its first lowering notch, operation remains the same as for lowering notch one of the controller HMC. When the controllers HMC, CMC are set to their third lowering notches, contactors HRC6, HRC7 remain open and the speedtorque curve is steep. When the controller CMC is on its fourth notch, however, the rotor resistance is reduced to flatten the speed-torque curve.